A general description of the sherardizing process can be found in numerous technical monographs or handbooks. See, for example, the monograph “Corrosion and protection of metals” by Bakhvalov and Turkovskaya, Pergamon Press, 1965.
The typical sherardizing process described in this monograph involves the following three main steps:                a) Cleaning of articles by sandblasting;        b) Loading of cleaned articles into closable receptacle together with a powder mixture, comprising zinc powder;        c) Thermal treatment of the receptacle within a furnace.        
The last step consists of slow heating of closed receptacle to 440° C. and holding thereof at that temperature for several hours.
There are known in the art various furnaces, in which the above-mentioned thermal treatment step can be carried out. For example, in RU2031186 and RU2130508 are disclosed periodically operating furnaces, in which closed receptacle is placed and heated. The receptacle is formed as elongated tubular, preferably cylindrical container. The parts to be coated are loaded within the container together with the zinc containing powder. During the thermal treatment the receptacle is rotated about its longitudinal or transversal axis. The advantage of the above-mentioned devices is associated with rotational movement of the receptacle. By virtue of rotation the powder more efficiently mixes with the loaded articles and this improves the homogeneity of diffusion and so the quality of the obtained coating. The disadvantage of the above-mentioned furnaces is associated with the fact that they operate periodically and therefore changing the amount of supplied thermal energy creates temperature gradient within the heating zone. This fact is associated with the necessity to wait until the process is fully completed before the new container enters the furnace. It can be readily appreciated, that the disadvantage of periodically operating furnace is associated with insufficient efficiency and capacity. Furthermore the maintenance of periodically operating furnace is complicated due to the necessity to repeat the same operations during each run.
In JP7173605 is described a furnace for carrying out of pack cementation. This furnace comprises an inlet chamber for loading containers with articles packed in the cementation powder, a heating chamber and a cooling chamber. The containers are configured as rectangular boxes, which are not tightly closed. Plurality of containers is transferred in sequence from the inlet chamber to the heating chamber and then to the cooling chamber. This furnace operates continuously, however it is not suitable for carrying out the sherardizing process, since it is designed to heat opened container, which is advanced strictly along the furnace and without a possibility for any additional movement.
In GB2082299 is described continuously operating heat-treatment furnace for pipes. This furnace comprises a furnace chamber for heating a plurality of pipes, arranged in parallel and transportable by a chain conveyor along the furnace. The pipes are oriented within the chamber in such a manner, that their longitudinal axes are perpendicular to the transport direction. There are provided plurality of stopper means for stopping the pipes intermittently at spaced-apart positions in the path of transport. The stopping causes each of the pipes to rotate about its own axis in co-operation with the chain conveyor. Pipes can be heat-treated in succession with a high efficiency. The force of transport of the chain conveyor causes the pipe to rotate once it reaches the stopper means and there is no necessity in any additional device for the rotation. In the above furnace the thermally treated objects are pipes opened from both ends and their rotation is utilized for preventing thermal deformation to an elliptical shape. Unfortunately the furnace, disclosed in the above patent is not designed and is not suitable for thermal treatment of parts, placed in sealed tubular containers, as it is required for thermal diffusion coating.
It should be pointed out that despite the process of thermal diffusion coating as such is known for a long time and there are known various furnaces, devised for carrying out this process, nevertheless there is still felt a strong need in a new and improved furnace, which is dedicated to this process and is free of the above-mentioned disadvantages of the known in the art devices.